The prior art reveals various methods for decreasing the distortion which results during the precipitation hardening of formed parts produced from beryllium copper alloys. Unfortunately, some of these prior art methods are only partially effective and often fail to minimize the resultant distortion to a commercially acceptable degree. Additionally, all of the prior art methods yield inconsistent non-reproducible results. These alloys are used in electrical connectors where reproducible dimensional, mechanical and electrical properties in the finished product are important if zero defects are to be obtained by the end user.
Basically, all prior art methods for producing precipitation hardened formed parts from beryllium copper alloys include the general combination of the following sequence of processing events: preparing a beryllium copper melt; casting the melt; hot working the cast alloy; solution annealing the alloy; cold working the solution annealed alloy; forming parts from the cold worked alloy; aging the cold worked beryllium copper. Various modifications have been developed in an attempt to minimize the non-reproducible dimensional and mechanical changes experienced in this processing sequence. The product of the general combination above or any of the modifications thereto will be termed broadly a processed beryllium copper alloy.
In this connection, reference is made to the methods disclosed in Guha U.S. Pat. No. 4,657,601; Inagaki U.S. Pat. No. 4,594,116; Woodard U.S. Pat. No. 4,579,603; Woodard U.S. Pat. No. 4,541,875; Rotem U.S. Pat. No. 4,533,412; Goldstein U.S. Pat. No. 4,425,168; McClelland U.S. Pat. No. 4,394,185; Wickle U.S. Pat. No. 4,179,314; Shapiro U.S. Pat. No. 3,882,712; Vernier U.S. Pat. No. 3,682,712; Chin U.S. Pat. No. 3,663,311; Britton U.S. Pat. No. 3,658,601; the article entitled "Residual Stresses in Copper--2% Beryllium Alloy Strips", authored by K. E. Amin and S. Ganesh, Experimental Mechanics, December 1981, page 474; the article entitled "A Technique for Predicting Distortion and Evaluating Stress Relief in Metal Forming Operations", authored by K. E. Amin and R. M. Rusnak, Journal of Metals, February 1981; the article entitled, "Stress Relaxation in Bending of Copper Beryllium Alloy Strip", authored by A. Fox in Journal of Testing and Evaluation, Vol. 8, No. 3, May 1980; the article entitled "Metallurgical Phenomena in High Strength Beryllium-Copper Alloys Which Affect Electrical Component Design", authored by John Ballance in 1Oth Annual Connector Proceedings, 1977; the article entitled, "Schrumpfung and Verzug bein Aus Harten von Kupfer Beryllium Legierungen", authored by H. Kreye, H. Noeka and F. Terline in Metall, 2 Jahrgang, November 1975; and the article entitled, "Precipitation Hardening in Cu 1.81% BE 0.28% Co" authored by W. Bonfield and B. C. Edwards in Journal of Materials Science, Vol. 9, 1974, page 406. The methods disclosed in these prior art sources are only partially successful in minimizing and making more reproducible the distortions in precipitation hardened finished products.
Kreye, Ballance, Bonfield and Edwards and others have shown the correlation between the shrinkage in strip and wire that occurs on aging and the decrease in volume due to the formation of the GuinierPreston zones and the gamma precipitates. To date, no one has presented evidence which demonstrates that the rate of formation of these G.P. zones and .gamma.", .gamma.' or .gamma. precipitates is different, when they are formed under compressive residual stresses, from the rate at which they are formed under tensile residual stresses. If it can be shown that the rates were dependent upon the stress system under which they occur, a lack of reproducibility in the residual stress patterns created during the forming of beryllium copper primary products and parts would mean a lack of reproducibility in the shrinkage created by the aging process. This would explain why it has been found that a decrease in the magnitude of the residual stresses results in a decrease in the magnitude of the non-reproducible shrinkage that occurs on aging. This application describes those techniques which will decrease the magnitude of the residual stresses, minimize the differences between the rates of formation of precipitates under tensile and compressive residual stresses and thus lead to the formation of mill hardened beryllium copper strip, wire, rod and tubing and parts formed from them that can be age hardened in a reproducible manner to give improved mechanical properties.
Amin and Rusnak have correctly identified residual stresses as one of the sources of distortion. Amin and Ganesh have also shown that a high rolling reduction of beryllium copper strip results in tensile residual stresses near the surface of the strip and compressive residual stresses at the center of the strip while low rolling reductions result in the opposite location of these stresses within the strip. The ability to create a reversal in the patterns of tensile and compressive residual stresses will be used to define the differences between heavy and light reductions. The results from Kreye et al. can be interpreted as showing that an irregular reduction, such as would result in bandoliered wire from hammer forging, will remove any such residual stress patterns in beryllium copper, particularly if the strokes are alternately light and heavy.
The Goldstein and the McClelland patents comprehend the importance of relieving residual stresses prior to the forming operation by the incorporation of a pre-aging technique. However, they fail to realize that in a thermal treatment, such as their pre-aging techniques, two reactions occur simultaneously. On the one hand, thermal treatments such as pre-aging reduce the magnitude of the existing cold working and residual stress patterns that affect the precipitation hardening. On the other hand, these treatments also promote the nucleation and growth of the precipitates formed during precipitation hardening. Fox has shown that in beryllium copper, stress relaxation and precipitation hardening can occur simultaneously at the same temperature. We have found that: the recognition of these competing mechanisms is critical in the development of reproducible softening and hardening techniques and the effects thereof on the formation of reproducible formed parts. That is, all thermal treatments must utilize those combinations of times, temperatures and heating rates that relieve or decrease the magnitude of residual stresses before the formation of precipitates become the dominant mechanism. This is illustrated in FIG. 1, and is discussed further hereinafter.
Chin teaches a process for obtaining an increase in yield strength and modulus of elasticity as well as an increase in formability for the phosphor bronze and cupro-nickel alloys. However, he failed to recognize that while he obtained similar increases in yield strength and modulus of elasticity for the beryllium copper alloys, his resultant lack of increased formability was due to his slow rate of heat up in the annealing step which followed the high reduction.
Ebert has taught that the increases in yield strength described by Chin are due to a decrease in the magnitude of the residual stresses. This he has done by showing that, based on the assumption of equal intensities of peak tension and compression residual stresses, the presence of compressive and tensile residual stresses create a drop in the yield strength and that, conversely, a decrease in the magnitude of these residual stresses increases the yield strength.
Ebert then attributes the increases in the yield strength and modulus of elasticity for beryllium copper, as shown in Chin's FIGS. 5 and 6, as being due to the decrease in the relative magnitudes of the residual stresses. These decreases are caused by the high strain reductions of over 85% as well as by the 2-hour anneals at temperatures ranging from 100.degree. F. to 525.degree. F. As is well known, these decreases in the magnitudes of the residual stresses occur at those temperatures and times which crete only minimal precipitation hardening. As will be shown in our Example 1, the decrease in residual stresses, before precipitation hardening becomes the dominant mechanism, is an important factor in creating beryllium copper primary products and parts that can be age hardened in a reproducible manner.
Part of the increase in the yield strength and formability of the nickel-rich beryllium copper alloys, as taught by Rotem, Inagaki and Guha, can be attributed to this decrease in the magnitude of the residual stresses. However, they do not indicate that this effect is a rate dependent phenomenon which can be improved by a fast heat up in the precipitation hardening step which follows their high reductions. This improvement in mechanical properties created by the increase in the heating rate will be shown in our Example 1.
None of the prior art teachings recognize that the rates at which the nucleation and growth of precipitates occur are different when the beryllium copper matrix is precipitation hardened under tensile residual stresses as opposed to compressive residual stresses. It has been shown that the formation of the gamma precipitates create shrinkage in the matrix as well as in the precipitates themselves. The formation of the G.P. zones and gamma precipitates result in a decrease in their volume from that of the solid solution. As the Be atoms diffuse out of the solid solution towards the G.P. platelet nuclei, the surrounding volume decreases. Then, it can be expected that the driving force for this diffusion should be different in a region where the lattice strains are decreased because they are under compression from that in a region where the lattice strains are increased under tensile residual stress.
Bonfield and Edwards have interpreted the actions of abutting G.P. zones, that are described by Phillips and Tanner, as indicating that the stress fields surrounding one zone produces cooperative interaction of adjacent zones to minimize the tensile and compressive residual stresses imposed on the matrix. While the magnitude of the unique effects of the G.P. zones on the mechanical properties of the beryllium copper alloys are recognized as being abnormally large, the role of residual stresses in the rate of formation of these zones has been totally ignored.
Also, what has not been recognized by the prior art is the existence of precipitate patterns, created during the post hot forming, cold forming and precipitation hardening operations by these differences in the rate of their formation. If there is a precipitate pattern, then when the precipitates are given the normal short time anneal, there will be left a residual pattern of concentrates of beryllium atoms and undissolved precipitates as well as reduced residual stresses. This residual pattern could form the basis for the memory, which becomes evident on aging, that the formed part has of its thermal and mechanical history. Evidence will be presented in Example 1 which shows that this memory can be minimized by the formation of an opposite pattern created by the second reduction and annealing taught herein.
The precipitate part of this memory can also be minimized by the short time high temperature anneal taught herein while the residual stress part of this memory can be minimized by the fast heat up of the alloy after a controlled high reduction step also taught herein. It is recognized that by the time that large non-coherent precipitates are formed, most of the effective residual stresses have vanished. However, the effects of this memory carry over during the formation of such non-coherent precipitates. In Example 3, the existence of such a memory will be confirmed and it will be shown how such a memory can be used to improve the offset yields in rolled strip.
If different contiguous volumes in a region of a beryllium copper part have different amounts of precipitates, these volumes will deform at different rates depending upon the relative size and number of their precipitates. That is, they will have different rates of work hardening. What has not been realized before is that the existence of precipitate patterns in beryllium copper created by the prior mechanical and thermal histories in the strip or wire, with their resultant differences in the rates of work hardening in different volumes, would make the beryllium copper shapes strain rate sensitive during cold forming operations. Further precipitation hardening would then increase such strain rate sensitivity. Until our test results have been described in Example 2, such strain rate sensitivity and its accompanying increase after precipitation hardening has not been recognized as existing in beryllium copper alloys nor any process developed to minimize the effect of this sensitivity on its mechanical properties.
In the extrusion of rod and in the rolling of strip of a strain rate sensitive material, it has been observed that at certain combinations of line speeds, reductions and die or roll diameters, regions of turbulence are carried through the die or between the rolls. The result is a lack of uniformity in the thickness and therefore in the amount of reduction created in the cold-rolled strip or wire. Minimizing these volumes of severe turbulence in such strip or wire would cause a more laminar flow condition and thus create a more uniformly thick strip or wire especially after aging. This is of importance to the production engineer. Then the more uniform the reduction along the length of the beryllium copper strip or wire, the more uniform would be the residual stress patterns set up along such strip or wire.
All these facts mean that to date there has been no application of (1) the effects of light and heavy reductions, (2) the minimizing of turbulence during cold forming, or (3) after a short time high temperature anneal, the severe reduction of the primary products followed by a low temperature rapid aging, to the minimizing and leveling out of the magnitude of the residual stresses within the primary products or to the minimizing of residual stresses set up within the alloy parts by such processes as forming, slitting, broaching, and machining. Once the importance of controlling the rate of precipitation and controlling the magnitude and rate of formation of residual stresses after the cooling operation are all recognized, it becomes possible by one skilled in the art to make those adjustments during and after hot forming that minimize the precipitate patterns.
With the foregoing in mind, it is a principal object of this invention to provide a process for relieving the magnitude of the residual stresses in beryllium copper alloys before the formation of precipitates becomes the dominant mechanism.
Another object of this invention is to provide a process for imposing on regions of compressive or tensile residual stress those tensile or compressive residual stresses that are of the opposite type.
Yet another object of this invention is to provide a process which makes more uniform the residual stresses of the reproducibly age hardenable primary products and thereby makes more reproducible the aging characteristics of parts that are machined, stamped or cold forged from such beryllium copper strip, wire, rod or tubing. Another object of this invention is to provide a process which minimizes the strain rate sensitivity of beryllium copper alloys through all stages of cold working and precipitation hardening.
Still another object of this invention is to provide a process which will result in a beryllium copper alloy which when aged exhibits an increased yield stress in tandem with an increased elongation.
Another object of this invention is to provide a process which improves the stress relaxation, the fatigue life and the high temperature properties of parts formed from reproducibly age hardenable strip, wire, rod or tubing.
Still another object of this invention is to provide a process which will minimize the effects of rolling, drawing, slitting or machining on the residual stresses created prior to aging regardless of the prior thermal and mechanical history of the part.
A further object of this invention is to provide a process for the virtual elimination of the non-reproducible part of the distortion which is currently experienced during the production of aged formed parts made from beryllium copper alloys.